Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack formed by stacking a plurality of fuel cells and a mounting section for mounting the fuel cell stack on the mounting section with inclination from a horizontal direction.
Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a power generation cell for generating electricity. In use, typically, a predetermined number (e.g., several hundreds) of the power generation cells are stacked together to form a fuel cell stack. For example, the fuel cell stack is mounted in a vehicle.
In the fuel cell, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas is formed on a separator surface facing the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas is formed on a separator surface facing the cathode.
In this system, water vapor is condensed into liquid water in the fuel gas flow field. Further, water is produced by power generation in the oxygen-containing gas flow field. The water tends to be retained in these flow fields. Thus, the fuel gas flow field and the oxygen-containing gas flow field may be clogged by the stagnant water retained in these flow fields. Under the circumstances, the fuel gas and the oxygen-containing gas may not be suitably supplied to the anode and cathode.
In this regard, for example, in a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2003-092130, as shown in FIG. 16, a cell assembly 2 formed by stacking a plurality of cells is provided in a body casing 1, and the body casing 1 is tiltable with respect to a reference plane h. An inlet port 3 for supplying a humidified gas to each gas flow field is formed at an upper position of the body casing 1, and a first outlet port 4 and a second outlet port 5 for the humidified gas discharged from the flow fields are formed at lower positions at both ends of the body casing 1.
When the body casing 1 is tilted with respect to the reference plane h, the water guided to a bottom surface 6 flows out of the second outlet port 5, and discharged to the outside of the body casing 1 through an open/close valve 7. According to the disclosure, no excessive water is retained in the gas flow field, and degradation of the power generation performance can be suppressed effectively.
In general, the fuel gas in the fuel gas flow field and the oxygen-containing gas in the oxygen-containing gas flow field flow in the same direction in a parallel manner or flow in the opposite directions in a counterflow manner. In the case where the fuel gas in the fuel gas flow field and the oxygen-containing gas in the oxygen-containing gas flow in a counterflow manner, the inlet and the outlet of the fuel gas flow field and the inlet and the outlet of the oxygen-containing gas field are provided opposite to each other. Specifically, the inlet of the fuel gas flow field and the outlet of the oxygen-containing gas flow field are disposed on the same side, and the outlet of the fuel gas flow field and the inlet of the oxygen-containing gas flow field are disposed on the same side.
In the structure, as in the case of Japanese Laid-Open Patent Publication No. 2003-092130, when the body casing 1 is tilted with respect to the reference plane h, water cannot be discharged smoothly from either the fuel gas flow field or the oxygen-containing gas flow field.
In particular, in the fuel gas flow field, in the case where pure hydrogen is used as a fuel gas, structure where a supply channel connected to the inlet side of the fuel gas flow field and a discharge channel connected to the outlet side of the fuel gas flow field are connected by an ejector is adopted to carry out supply of the fuel gas by circulation. In this type of the system, since the flow rate of the circulating fuel gas is not sufficient, the condensed water tends to be retained in the fuel gas flow field. Therefore, the flow resistance is increased by the retained condensed water, and the fuel gas is not sufficiently supplied to the reaction surface. Thus, stoichiometric ratio of the fuel gas is reduced, and the power generation performance is degraded. The stoichiometric ratio of the fuel gas is regarded as being lowered when the amount of fuel gas supplied becomes even smaller than the amount of fuel gas actually required.
Further, in the system where a pump for forcibly circulating hydrogen is provided, in order to solve this problem, a pump having good performance may be employed as the pump for forcibly circulating hydrogen. However, since the density of the hydrogen is small, the load on the pump becomes significantly high, and the efficiency in the overall system becomes very poor.